Société nationale de protection de la nature et d'acclimatation de France, Paris (FRA)
Abstract
Dans le contexte de pullulation généralisée des rongeurs sahéliens en 1975-1976, les auteurs ont recueilli des informations sur l’installation, puis le maintien d’une population d’ Arvicanthis niloticus dans le milieu naturel du Sahel sénégalais. L’invasion consécutive à la phase d’expansion maximale des populations a eu lieu durant la saison des pluies 1975. Les Arvi canthis se sont installés dans les formations arbustives des dépres sions interdunaires. Pour faire face aux rigueurs de la saison sèche, les Arvican this ont modifié leur comportement diurne, qui de terrestre est devenu arboricole : les arbres leur offrant par leurs écorces, les ressources hydriques indispensables à leur survie. Cette exploi tation inhabituelle de la strate arborée aboutit à une dégradation empêchant la régénération d’espèces comme Commiphora afri- cana et Acacia senegal. Les caractéristiques de la dynamique de population des Arvi canthis suggèrent que le maintien dans le milieu naturel est dif ficile : la longueur de la saison de reproduction est suffisante pour permettre l’apparition d’une deuxième génération issue des premiers nés, mais la durée de vie, qui ne dépasse pas 9 mois en raison des difficultés alimentaires et de la pression de prédation, réd.uit considérablement les possibilités de maintien à long terme.A rodent outbreak, involving several species, took place in 1975-1976 in the sahelian zone of Senegal. This first paper des cribes the population movements and fluctuations of the Nile rat, Arvicanthis niloticus in the Ferlo, particularly on the IRP quadrat of Fété-Olé. In Northern Senegal the Nile rat lives permanently only in the more humid and cultivated areas, and around human settle ments. Here, their numbers most likely began to increase after the 1974 rains, following the severe 1972-1973 drought. As in the other species of rodents, this situation led to a lengthening of the breeding season and an increased fecundity among the Nile rats which started invading the sahelian savanna in June-July 1975. During the 1975 rains the rat invaders started to breed again and the subsequent population increase led to the 1975-1976 outbreak. The Nile rat pullulation came to its end at the close of the next dry season, in April-May 1977. The living conditions of Arvicanthis niloticus in the sahelian savanna are described. The rats settled mostly in the more den sely wooded depressions between fossil dunes, where temporary pools occur after the rains. Here they became active by day and foraged in trees and bushes. They ate shoots and bark, mostly from Acacia Senegal and Commiphora africana, and shoots, leaves, flowers, fruits and bark of Balanites aegyptiaca. The highest densities were estimated to be close to 100 Nile rats/ha in February 1976, and the lowest 17.5 rats/ha in June 1976. Population numbers, early in 1977, did not exceed 35.1 Nile rats/ha. The study of Arvicanthis niloticus population structure and dynamics suggests that the species’ prospects of settling perma nently in the sahelian savanna are poor. The breeding season of the species is short (5 months) and the population turnover rapid (average life expectancy : 9 months). A high predation pressure and a shortage of food and water may be held responsible for the inability of the species to colonize permanently the sahelian savanna. The trophic impact of the Nile rat outbreak upon the tree stratum of the Fété-Olé IBP quadrat has been estimated. During the 1975-1976 dry season, 97 % of the Commiphora africana shoots were destroyed. As for Acacia senegal, 80 % of the trees were damaged in 1975-1976, and 33 % in 1976-1977
